Cuspidal divisor class groups of non-split cartan modular curves

I find an explicit description of modular units in terms of Siegel functions for the modular curves X+ ns(pk) associated to the normalizer of a non-split Cartan subgroup of level pk where p 6= 2,3 is a prime. The Cuspidal Divisor Class Group C+ ns(pk) on X+ ns(pk) is explicitly described as a module over the group ring R = Z[(Z/pkZ)∗/{±1}]. In this thesis I give a formula involving generalized Bernoulli numbers B2,χ for |C+ ns(pk)| and estimate the order ofgrowth of |C+ ns(p)|. I study the p−primary part of the Cuspidal Divisor ClassGroup at level p.